As rolling bearings to be used under severe conditions such as high temperature, corrosive environment, vacuum or non-lubricant atmosphere in which normal metallic material cannot be used, there can mention so-called ceramic bearings wherein at least a rolling element (and more preferably, a rolling element and a race) is/are made of a ceramic material.
Such ceramic material preferred to use are those prepared by sintering a sintering material containing particles of silicon nitride(Si.sub.3 N.sub.4) because they are excellent in heat resistance and wear resistance.
Silicon nitride itself is, however, relatively poor in sintering properties. Therefore, to a sintering material, a variety of sintering assistants which improve sintering properties of the silicon nitride are generally added in each desired amount.
Examples of conventional sintering assistants include rare earth oxide such as yttrium oxide(Y.sub.2 O.sub.3), aluminum oxide (Al.sub.2 O.sub.3) and aluminum nitride(AlN), among others.
As exemplary sintering material of silicone nitride containing the sintering assistants as described, those of Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --AlN are known in the art.
In the ceramic bearings prepared from the sintering material of silicon nitride, however, particularly sintering assistants and more particularly yttrium oxide is corroded when used in a corrosive atmosphere such as a strong acid atmosphere and a strong alkali atmosphere, reducing bonding strength between silicon nitride particles. This has been causing the problem that the strength of a ceramic bearing significantly reduces so that breakage occur before its design life.
It is accordingly the object of this invention to provide rolling bearings of a long service life, which are excellent in corrosion resistance when used under a corrosive atmosphere, as well as heat resistance and wear resistance.